Thin polymerized layers can be generated from organic vapors subjected to glow discharge, and the process is usually called "plasma polymerization". In general, the plasma polymerized layers have some interesting properties, and no catalyst is required in the processing. The emphasis has been on layers produced from hydrocarbon based systems, however, this trend in more recent times has swung towards fluorocarbons.
In most cases electric power generators at radio frequency are utilized to generate plasma. When an organic vapor is introduced into plasma, the monomer gains energy from the plasma through inelastic collision and is fragmented into activated small molecules. These small molecules are recombined to form a large molecule (polymer) and deposited at the surface of a substrate. In this process, the growth of monomers into polymers occurs with the assistance of plasma energy, which involves activated electrons, ions, and radicals.
Plasma polymerization is a thin layer-forming process. The unique feature of plasma polymerization is the formation of a ultra-thin layer with a minimal amount of flaws. The layers usually are cross-linked, insoluble, pinhole-free and heat resistant. With respect to spin coating or vacuum deposition plasma polymerization provides layers with excellent conformality, sufficient durability, and improved adhesion.
Plasma-polymerized thin organic layers are, in general, dielectric materials with insulating properties and extremely low conductivities. For tetrafluoroethylene polymerized by plasma, Vollman and Poll reported dc conductivity values in the range 10.sup.-17 -10.sup.-18 (ohm-cm).sup.-1. Hetzler and Kay (see Plasma Polymerization by H. Yasuda) reported ac conductivities between 10.sup.-15 and 10.sup.-10 (ohm-cm).sup.-1 measured at 10.sup.-3 -10.sup.5 Hz. For very low frequencies, however, the conductivity leveled off to the dc value at 10.sup.17 (ohm-cm).sup.-1, which is in good agreement with Vollman and Poll's data Kay (see Plasma Polymerization by H. Yasuda). Because of the low conductivities, the materials are commonly used as dielectrics or corrosion protective coating.
Doping of acceptor or donor molecules into plasma polymerized layers has been attempted to improve their conductivities, but the results are not successful. Plasma polymerized layers with high electronic conductivities could only be formed from some specific monomers, such as organogermanium or organometallic compounds. Kny et al. Kay (see Plasma Polymerization by H. Yasuda) reported the formation of conductive polymers using the plasma polymerization of tetramethyltin. The magnitude of the conductivity strongly depends on the amount of Tin present in the polymer layers.
The study of conductive polymers started from 1977. It was first reported that polyacetylene, a conjugated organic polymer, could attain high levels of electronic conductivity when oxidized by suitable reagents. The concept of conductivity and electroactivity of conjugated polymers was quickly broadened from polyacetylene to include a number of conjugated hydrocarbon and aromatic heterocyclic polymers, such as poly(p-phenylene), poly(p-phenylene vinylene), poly(p-phenylene sulfide), polypyrrole, and polythiophene, while no success was achieved with fluorocarbon polymers. The principal methods to prepare conducting polymers include electrochemical oxidation of resonance-stabilized aromatic molecules, structure modification along with doping, and synthesis of conducting transition metal-containing polymers. The presence of a large number of conductive polymers has led to a number of potential application. For instance, polymer light-emitting diodes with enhanced performance were fabricated using polyaniline (PANI) network electrodes. The metallic emeraldine salt form of PANI is prepared by protonation with functionalized sulfonic acids, yielding a conducting PANI complex soluble in common organic solvent. However, because of the process complexity and the nature of spinning coating it is not compatible to the fabrication of organic electroluminescence (EL) devices.